by law, fbi officials said, monitoring had to stop, barring new negative information. >> too many people in this country, just like him, that are touching extremist websites, that are espousing things that aren't particularly kind to americans, but that's not against the law here. >> reporter: tamerlan's father said his sons couldn't have planned the attack, because they knew the fbi was watching them. but they apparently were not. and never knew that a dark side of the two brothers had begun to emerge. tamerlan was charged in cambridge with beating his former girlfriend. he started posting violent videos, urging jihad. on his youtube channel. and dzhokhar began writing bizarre tweets. "a decade in america already, i want out." authorities never saw this coming. tamerlan traveled to russia for six months last year. the fbi wants to know exactly who he saw and what he was doing, dan. >> let me get back to the fbi investigation into the older brother. you've been talking to the fbi all day. are they not expecting any sense of frustration or remorse over having missed what could have been a